1. Field of the Invention
The present invention relates to an optical scanning device and an image forming apparatus.
2. Description of the Related Art
A conventional technology for scanning a surface of a document by different beams emitted from a common light source using a pyramidal mirror or a plane mirror is disclosed in Japanese Patent Application Laid-Open No. 2002-23085.
Although the number of the light sources can be decreased by using the common light source with the technology described above, it is difficult to increase an operation speed because the number of surfaces of a deflecting mirror cannot be increased more than two.
A tandem-type image forming apparatus including a plurality of photosensitive elements (normally, four photosensitive elements) is increasingly commercialized because an electrophotographic image forming apparatus, such as a laser printer, a digital copying machine, a plain-paper facsimile, is improved to form a color image with an increased operation speed. An electrophotographic color image forming apparatus can be configured in such a manner that the photosensitive elements rotate as many times as the number of necessary colors (e.g., if one drum is provided for four colors, the drum needs to rotate four times), which may cause a poor productivity.
With the tandem-type image forming apparatus, the number of light sources needs to be increased, increasing the number of necessary components. As a result, a color misalignment is likely to occur due to a difference in wavelengths of the light sources, and more costs are required. In addition, a failure may occur in a writing unit due to a degradation of the light source such as a semiconductor laser. When the number of the light sources is increased, the possibility of the occurrence of a failure is increased, resulting in a difficulty in reuse of the components.
A technology employing a single light source is disclosed in Japanese Patent Application Laid-Open No. 2005-92129, in which a beam emitted from the single light source is split and each of split beams is led to a different scanning target surface.
Even though the number of the light sources can be decreased with the above technology, although a high-speed output of an image can be achieved, if a single synchronization detection sensor obtains synchronous signals for two stations or more, the stations cannot be electrically distinguished from each other because a time taken for scanning each of the stations is identical. As a result, it is difficult to perform a feed back control for correcting a timing of starting a writing for each station.